In vivo 5-bromodeoxyuridine (BbrdU)-dye methodology is utilized for a variety of cytogenetic studies in rodent germ cells. Analyses of spermatogonial DNA replication and sister chromatid exchange (SCE) patterns are accomplished, and meiotic exchanges are visualized in 1 degree and 2 degree spermatocytes. Together, these studies can form a system for interpreting details of meiotic recombination. Mutagen effects upon SCE and/or meiotic exchange will be examined.